The present invention relates generally to high power semiconductor switching devices and, in particular, to thyristors with trench emitter control.
The utility of thyristors is well known. Thyristors are usually four layer devices, each adjacent layer having an opposite conductivity type from its adjacent layers. As known to those skilled in the art, a simple thyristor generally includes a four-layer P1-N1-P2-N2 device with three P-N junctions in series, J1, J2 and J3 respectively. The four layers correspond to the anode (P1), the first base region (N1), the second base (or P base) region (P2) and the cathode (N2) respectively. In the forward blocking state, the anode is biased positive with respect to the cathode and junctions J1 is forward biased and J2 and J3 are reverse biased and most of the forward voltage drop occurs across the central junction J2. In the forward conducting state, all three junctions are forward biased and the voltage drop across the device is very low, approximately equal to the voltage drop across a single forward biased P-N junction.
An inherent limitation to the use of thyristors for high current applications is sustained latch-up, generally arising from the action of the coupled P1-N1-P2 and N1-P2-N2 bipolar transistors which make up the four layers of the thyristor. Destruction of the thyristor can occur if the latch-up current is not sufficiently controlled by external circuitry or by reversing anode potential. Alternatively, the latch-up current can be controlled by the use of a MOSFET or similar device for controlling turn on and turn off.
Several methods exist for obtaining MOS gate control over thyristor action. One of the more successful methods is a MOS-controlled thyristor, in which turn off is provided by shorting the emitter-base junction of the N-P-N transistor to thereby reduce the gain of the device. For example, an MCT may include an P-channel MOSFET integrated into the cathode region of the thyristor for turn-off control and an N-channel MOSFET integrated into the P Base region for turn on control. (See, for example, the article published by Victor A. K. Temple, published in IEDM Technology Digest, pp. 282-285 (1984).)
Still, it is desirable to obtain power switch devices which can conduct more current than prior devices and yet provide reliable control and safe operating area. Additionally, such devices can be improved if the devices have a higher current saturation capability and smaller temperature dependence. One such device which has recently been studied is an Emitter Switched Thyristor ("EST") as shown in FIG. 5. In the EST 10, a four layer structure is used including a collector region 12, a drift region 14, a body region 16, and a floating region 18. At the top of the device an emitter 20 is placed over and in the body region 16. A turn on gate 22 spans the portion of the body region 16 between the floating region 18 and the drift region 14. A turn off gate 24 spans the portion of the body region 16 between the emitter 20 and the floating region 18. As is well known, each of the gates operates selectively to establish a channel within the adjacent body portion 16 or the gates are tied together to form a threes terminal device. The EST is known to have a relatively large safe operating area and a fast switching spesed. The on-voltage of the EST is, however, about a half-volt higher than that of an MCT and is caused primarily by the voltage drop on the lateral MOSFET channel resistance. Since the on-voltage of the device represents its efficiency, it is highly desirable to obtain a device with similar properties but with a lower on-voltage . In addition, it is known that EST devices generally suffer from turn-on problems generally relating to the fact that the turn-on channel density is small because of the long floating N+ region's consumption of silicon area.
It is accordingly an object of the present invention to provide a thyristor which can be readily and effectively controlled.
It is another object of the present invention to provide a novel semiconductor device which can reliably switch relatively large amounts of voltage with a relatively small on-voltage.
It is still another object of the present invention to provide a novel semiconductor device which has a higher electron current density so that it will readily turn-on into the thyristor mode.
It is yet another object of the present invention to provide a novel semiconductor device and circuit which obviates the foregoing problems of power switching devices but provides comparable power switching capability with a lower forward voltage drop.
It is still a further object of the present invention to provide a novel semiconductor device which can safely operate as a thyristor and which can be readily fabricated using current MOSFET manufacturing processes.
These and other objects of the present invention will be known to those skilled in the art from the following descriptions of a preferred embodiment in view of the drawings.